What Gears for a TH350 Trans

[Grumpyroger]

I am a Newbie here, and I have a small dilemma.... I own a 1969 Chev. C-20 equipped with a 350ci engine, I believe a TH350 trans (although the vacuum modulator is located on the right front of trans) and 4.10 rear end gears. It came with 16.5 tires and split rims, but I have changed them to 245/75/16 (30" tall) My problem is trying to figure out which gears would be better suited for Highway use. With the 4.10 gears I turn about 3500 RPM at 65-70 MPH and get real crappy fuel mileage. Any suggestions ?

[haulnrat]

3.07 is what is in most half tons. That works pretty well for highway and most uses. A 3.42 would probably be the most I would drop to, depending on what you want to use it for.

[mr48chev]

I run pretty much the same setup in my 3/4 ton. Same engine, trans gear ratio an tire size.
As my truck likes to cruise right about 58 mph with the 4.11 I am going to put in the 3.55 Posi that I bought a while back to get the engine rpm down to the same as it now as at 57/58 but running at 70 on the freeway.

Here is a pretty easy to use speed x rpm calculator that I use quite a bit.
http://www.csgnetwork.com/multirpmcalc.html

Here is one that is fun to play with but the options are limited.
http://vexer.com/automotive-tools/speed-rpm-calculator

A bit of a different one from Wallace Racing
http://www.wallaceracing.com/gear-speed.php
You have to scroll down to see the speeds.
They also have calculators for a lot of car/truck stuff by clicking the back to calculators link.

[Longhorn Man]

if you have the eaton rear, the 4.10 is as good as you'll get. Time to decide what kind of rear you want